Pipe Thread Tap Drill Chart (NPT / BSPT / BSPP)

Tap drill sizes for tapered pipe threads (NPT, NPTF, BSPT) and parallel pipe threads (NPS, BSPP / G). Includes nominal size, OD, threads-per-inch, and drill size in inches and millimetres.

Reference Reference Updated Apr 24, 2026
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Pipe threads use different tap drills than machine-screw threads because the thread is tapered (3/4 inch taper per foot on NPT / BSPT). NPT and NPTF thread sealing relies on metal-on-metal interference at the crests; use thread sealant or PTFE tape. BSPP (also called "G") is a straight thread that seals on a gasket or O-ring, not by thread interference. Match thread standards strictly — NPT and BSPT look similar but have different angles (60° vs 55°) and are not interchangeable.

NPT — National Pipe Taper (ANSI B1.20.1)

Nominal OD (in) TPI Tap Drill (in) Decimal Tap Drill (mm)
1/16" 0.3125 27 C (0.242) 0.242 6.15 mm
1/8" 0.4050 27 11/32 0.344 8.74 mm
1/4" 0.5400 18 7/16 0.438 11.11 mm
3/8" 0.6750 18 37/64 0.578 14.68 mm
1/2" 0.8400 14 23/32 0.719 18.26 mm
3/4" 1.0500 14 59/64 0.922 23.42 mm
1" 1.3150 11.5 1-5/32 1.156 29.37 mm
1-1/4" 1.6600 11.5 1-1/2 1.500 38.10 mm
1-1/2" 1.9000 11.5 1-47/64 1.734 44.05 mm
2" 2.3750 11.5 2-7/32 2.219 56.36 mm
2-1/2" 2.8750 8 2-5/8 2.625 66.68 mm
3" 3.5000 8 3-1/4 3.250 82.55 mm
3-1/2" 4.0000 8 3-3/4 3.750 95.25 mm
4" 4.5000 8 4-1/4 4.250 107.95 mm

NPTF — Dryseal (ANSI B1.20.3)

  • NPTF uses the same tap drill sizes as NPT but with tighter tolerance on thread crests and roots — seals without PTFE tape.
  • Requires NPTF-specific taps (dryseal); regular NPT taps will not produce a spec NPTF thread.
  • Drill the hole exactly as the NPT chart above, then reamer-ream for best finish before tapping.

BSPT — British Standard Pipe Taper (ISO 7 / R-series)

Nominal OD (mm) TPI Tap Drill (mm) Tap Drill (in)
R 1/16 7.723 28 6.5 mm 0.256 (≈ F)
R 1/8 9.728 28 8.8 mm 0.346 (≈ S)
R 1/4 13.157 19 11.8 mm 0.465 (≈ 15/32)
R 3/8 16.662 19 15.25 mm 0.600 (≈ 19/32)
R 1/2 20.955 14 19.0 mm 0.748 (≈ 3/4)
R 3/4 26.441 14 24.5 mm 0.965 (≈ 31/32)
R 1 33.249 11 30.75 mm 1.211 (≈ 1-7/32)
R 1-1/4 41.910 11 39.5 mm 1.555 (≈ 1-35/64)
R 1-1/2 47.803 11 45.25 mm 1.781 (≈ 1-25/32)
R 2 59.614 11 57.0 mm 2.244 (≈ 2-1/4)
R 2-1/2 75.184 11 72.5 mm 2.854 (≈ 2-27/32)
R 3 87.884 11 85.0 mm 3.346 (≈ 3-11/32)

BSPP / G — British Standard Pipe Parallel (ISO 228)

Nominal OD (mm) TPI Tap Drill (mm) Tap Drill (in)
G 1/16 7.723 28 6.8 mm 0.268
G 1/8 9.728 28 8.8 mm 0.346
G 1/4 13.157 19 11.8 mm 0.465
G 3/8 16.662 19 15.25 mm 0.600
G 1/2 20.955 14 19.0 mm 0.748
G 5/8 22.911 14 21.0 mm 0.827
G 3/4 26.441 14 24.5 mm 0.965
G 1 33.249 11 30.75 mm 1.211
G 1-1/4 41.910 11 39.5 mm 1.555
G 1-1/2 47.803 11 45.25 mm 1.781
G 2 59.614 11 57.0 mm 2.244

NPS — National Pipe Straight (ANSI B1.20.1)

  • NPS has the same TPI and OD as NPT but is parallel — used for mechanical connections (fittings, bushings), not sealing.
  • Use the NPT tap drill sizes; you may want to ream 0.005" oversize for slip fit.
  • NPSM (mechanical), NPSL (locknut), and NPSH (hose) are all variants with the same pitch but different tolerance classes.

Usage notes

  • Tap taper pipe threads with a taper tap only (pipe taps are self-starting and cut full profile). Never run a plug or bottoming tap after — you'll damage the seal.
  • Go one full turn past finger-tight after assembling NPT fittings, then 1½-2 turns more with a wrench. Over-tightening splits fittings.
  • PTFE tape: wrap 2-3 turns in the direction of thread rotation on the male thread. Leave the first 1-2 threads bare to prevent shavings getting into the system.
  • BSPP (G) threads seal on a face — add a bonded washer or O-ring on the male side. The thread itself does no sealing.
  • Never mix NPT and BSPT on the same joint; different thread angles (60° vs 55°) give only 3-4 contact threads which leaks fast.

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